236 CONTROL MECHANISMS IN CELLULAR PROCESSES 



24 hours. Note also that we have inckided a vial in which FUDR 

 was added but not removed. With some compounds this required 

 using a considerably lower concentration than that which could be 

 used in a pulse. 



D|8 L- 06 DlM-^iQ 06 



"iS 56 



cent, confrol 

 -»uncen+. control 



FUDR IQ-^ M 



light 1000 f.c. 

 uncent. control 



18 



Fig. 9-5. This and the subsequent figures show many cultures in each figure, 

 all at 24° C. Each culture is traced along a horizontal line, and the times at 

 which maxima occurred on successive days are indicated by black triangles. 

 All cultures were placed in constant dim light subsequent to exposure to light- 

 dark cycles (12 hours light; 12 hours dark), which ended at last dark period 

 (black rectangle on abscissa). In this figure are shown phase shifts resulting 

 from exposures to bright light at difFerent times of day. The times at which 

 these light treatments were given are indicated by bars along the horizontal 

 lines. Note that the phase shift is highly dependent upon the time of day when 

 light is administered. Chemical "pulses" are indicated in a similar way, the time 

 period of exposure being indicated by a bar on the horizontal line. Where the 

 compound was not removed, the bar continues. The compound and its approxi- 

 mate final concentration is indicated to the right. Centrifuged controls are vials 

 to which distilled water instead of the compound was added, and centrifugation 

 and resuspension in fresh medium was carried out as usual. FUDR = 5'-fluoro- 

 S'-deoxyuridine. 



With FUDR the glow maxima occur very much as in the controls. 

 There may be a slight shift of some 1 to 3 hours. Whether or not 

 this is significant is not known, but we are inclined to think that 

 it is not. 



In these experiments we have attempted to characterize the 

 actual effect that the compound has upon the biochemical processes 



